The present invention relates to a multilayer filter that exhibits nondecreasing efficiency when challenged with an oily-mist aerosol.
Persons who are exposed to air that contains toxic or noxious substances frequently wear a respirator that covers their nose and mouth to filter air before it is inhaled. Respirator design is regulated by the National Institute for Occupational Safety and Health (NIOSH). NIOSH establishes various removal efficiency standards for respirators that have been challenged with different contaminants. For example, a standard for oily-mist removal efficiencyxe2x80x94using suspended droplets of dioctylphthalate (DOP)xe2x80x94must be met by filter media designated for use in environments where oil is present. Because removal efficiency may change in response to loading, the standards specify a minimum removal efficiency over a fixed exposure to a challenge aerosol. Although some respirator filter media provide nondecreasingxe2x80x94or even increasingxe2x80x94efficiency with continued exposure, removal efficiency of respirator filter media typically decreases as DOP exposure increases.
Effective July 1995, NIOSH instituted standards for nonpowered air-purifying particulate respirators. See 42 C.F.R. Part 84 (published Jun. 8, 1995). The regulations include several different classifications, one of which is commonly referred to as xe2x80x9cP-seriesxe2x80x9d and is directed at filters that are intended for removal of oil-based liquid particulates. For a P-series certification, the respirator filter media must exhibit nondecreasing efficiency at the end point of a DOP removal efficiency test.
In addition to removal efficiency, respirator comfort is another parameter that is important to the respirator user. One indicator of respirator comfort is pressure-drop across the respirator filter media. Lower pressure-drop filters provide improved comfort to users by making it easier for the wearer to breathe and by allowing warm, moist exhaled-air to be more easily purged from a mask that does not possess an exhalation valve.
Although pressure drop is not substantially affected by environmental conditions, comfort differences between filters that have high and low pressure-drops do, however, become more pronounced under more severe environmental conditions, such as high heat and humidity. Under these conditions, the perceived pressure-dropxe2x80x94that is, the perceived effort to breathe through the respiratorxe2x80x94can increase to uncomfortable levels. Perceived pressure-drop increases are a function of environmental conditions because users in more severe environmental conditions typically are more aware of the additional effort required to breathe than users in less severe environmental conditions. The increase in perceived pressure-drop reduces a user""s comfort and may lead to reduced compliance with respirator use requirements, particularly in the more severe environmental conditions where respirator use is very important to a person""s health and safety.
Pressure-drop can also be an important factor for filter media that is used in powered air-purifying respirators. Unlike negative pressure respirators, which rely on a wearer""s lungs to draw air through the filter media, powered air-purifying respirators use an external power source to accomplish this task. Powered air-purifying respirator performance is measured by a number of parameters, including airflow and pressure-drop across the filter during operation. Airflow and pressure-drop are related because, for a given blower and power source, a filter that has a lower pressure-drop will deliver a higher airflow. Conversely, a filter with a higher pressure-drop will deliver lower airflow using the same blower and power source. Airflow and pressure-drop are important because a respirator system that has a higher pressure-drop filter requires more energy to deliver the same amount of filtered air than a respirator system that has a lower pressure-drop filter. As a result, a higher pressure-drop can result in reduced operating times for powered respirator systems that have fixed energy sources such as batteries.
Pressure-drop for a given airflow rate across a filter can be decreased by increasing the openness or looseness of the filter material. A filter in which the openness or looseness of the filter material is increased, however, typically exhibits reduced efficiency in removal of contaminants, which is yet another parameter by which respirator system performance is measured. Pressure-drop for a given airflow rate, in some circumstances, can also be reduced without decreasing the contaminant removal efficiency. This can be accomplished by increasing the size or surface area of the filter. Increasing filter size, however, typically also includes increasing the size and/or bulk of the system, which may potentially limit the wearer""s mobility in confined areas.
Attempts to meet the need for P-series certified filters typically have relied on the use of filter media that exhibits nondecreasing removal efficiency. Some filters may include one or more layers that exhibit neutral removal efficiency in combination with one or more layers that exhibit nondecreasing removal efficiency to provide filters that exhibit nondecreasing removal efficiency. The neutral removal efficiency layer may be used as a prefilter to prevent caking of the filter layer(s) that provide nondecreasing removal efficiency and are typically selected for their low pressure drop characteristics. To provide a low pressure drop, the prefilter layers may require relatively high loft, which can substantially increase the filter thickness. As discussed above, increased filter thickness may increase the size of the filter system, potentially limiting the wearer""s mobility in confined areas.
What is needed are filters for use in respirators and other articles that are able to meet the NIOSH P-series oily-mist removal efficiency requirements while also providing reduced pressure-drop.
The present invention can provide an oily-mist resistant filter that has a nondecreasing removal efficiency in combination with a reduced pressure-drop. The filter can offer improved wearer comfort when used in non-powered air-purifying respirators.
The filters of the invention offer these advantages by providing a new electret filter that comprises a fluid permeable first electret filter layer that exhibits nondecreasing removal efficiency at completion of the DOP Penetration/Loading Test and removes a majority of a challenge aerosol collected by the filter during the DOP Penetration/Loading Test. The electret filter also comprises a fluid permeable second electret filter layer that exhibits decreasing removal efficiency at completion of the DOP Penetration/Loading Test. The second electret filter layer also exhibits an initial quality factor that is greater than an initial quality factor of the first electret filter layer as determined using the DOP Penetration/Loading Test. The second electret filter layer is located downstream to the first layer when viewed in the direction of the fluid flow. Both the first and second filter layers include fibers that contain polymeric materials.
The present invention differs from known filters by providing a first layer, which has a nondecreasing removal efficient and an ability to remove a majority of the challenge aerosol, upstream to a second layer that has a decreasing removal efficiency and an initial quality factor greater than the first layer. This new combination of filter layers can allow the filter as a whole to exhibit a nondecreasing removal efficiency at the completion of the DOP Penetration/Loading Test. This performance feature can enable the filter to satisfy the NIOSH P-Series standard for oil-based liquid particulates. Filters of the invention also can be capable of providing a relatively low pressure drop. The invention thus is beneficial for respirators in that it can provide a very safe breathing environment to the user in an oily-mist environment; while at the same time providing good comfort to the wearer by virtue of a low pressure drop.
In reference to the invention, the following terms are defined as set forth below. Other terms may also be defined with reference to the specification, claims, and drawings.
xe2x80x9cAerosolxe2x80x9d means a gas that contains suspended particles in solid or liquid form;
xe2x80x9cbasis weightxe2x80x9d means the weight of the material or materials in a layer per unit surface area of the major surfaces of the layer;
xe2x80x9cdecreasing removal efficiencyxe2x80x9d means that the filter or filter layer exhibits decreasing removal efficiency as indicated by a positive slope in the DOP Percent Penetration curve at completion of the DOP Penetration/Loading Test described in the test section below (where completion occurs at a total exposure of 200xc2x15 milligrams DOP);
xe2x80x9cefficiencyxe2x80x9d means the amount, expressed in percent, of a challenge aerosol removed by a filter, which can be determined based on percent penetration where
efficiency(%)=100xe2x88x92penetration(%)
(for example, a filter exhibiting a penetration of 5% would have a corresponding efficiency of 95%);
xe2x80x9celectric chargexe2x80x9d means that there is charge separation.
xe2x80x9celectret filterxe2x80x9d or xe2x80x9celectret filter layerxe2x80x9d means a filter or filter layer that exhibits at least quasi-permanent electrical charge, where xe2x80x9cquasi-permanentxe2x80x9d means that the electric charge resides in the web under standard atmospheric conditions (22xc2x0 C., 101,300 Pascals atmospheric pressure, and 50% humidity) for a time period long enough to be significantly measurable;
xe2x80x9cfirst layerxe2x80x9d means the layer of the filter that is encountered by an airstream before the second layer;
xe2x80x9cfluid permeablexe2x80x9d means that the filter or filter layer permits the passage of at least a portion of a fluid;
xe2x80x9clayerxe2x80x9d means a portion of a filter that has two major surfaces and a thickness between the major surfaces, the layer may extend for an indefinite distance along the major surfaces or it may have defined boundaries;
xe2x80x9cmajorityxe2x80x9d means more than 50%;
xe2x80x9cmelting temperaturexe2x80x9d is determined by differential scanning calorimetry (DSC) conducted at a heating rate of 10xc2x0 C./min, and defined as the peak maximum caused by melting that is observed in the second DSC heating cycle (i.e. the peak observed after heating to above the melting temperature, cooling to freeze the article and reheating);
xe2x80x9cnondecreasing efficiencyxe2x80x9d and xe2x80x9cnondecreasing removal efficiencyxe2x80x9d mean that the filter or filter layer exhibits nondecreasing removal efficiency as indicated by a non-positive slope in the DOP Percent Penetration curve at completion of the DOP Penetration/Loading Test described in the test section below (where completion occurs at a total exposure of 200xc2x15 milligrams DOP);
xe2x80x9coily-mist performance-enhancing additivexe2x80x9d means an additive that, when provided as part of the filter, improves the ability of the filter to capture oily-mist particles;
xe2x80x9cpolymerxe2x80x9d means a macromolecule made from monomers and includes homopolymers, copolymers, and polymer blends;
xe2x80x9cpolymeric materialxe2x80x9d means material that includes at least one polymer and possibly other ingredients in addition to a polymer;
xe2x80x9cpressure-dropxe2x80x9d means a reduction in static pressure within an airstream between the upstream and downstream sides of a filter through which the airstream passes;
xe2x80x9crespiratorxe2x80x9d means a system or device worn over a person""s breathing passages to prevent contaminants from entering the wearer""s respiratory tract and/or protect other persons or things from exposure to pathogens or other contaminants expelled by the wearer during respiration, including, but not limited to filtering face masks; and
xe2x80x9csecond layerxe2x80x9d means the filter layer that is encountered by the airstream after passing through the first layer.